The present invention relates to a scanning optical system having two lens elements including at least one plastic lens element, surfaces of the two lens elements being processed to prevent a ghost due to reflection of light between surfaces.
A scanning optical system employed in a laser beam printer, a laser scanner or a bar code reader is typically configured such that a laser beam emitted by a light source is deflected by a deflector such as a polygonal mirror, which is rotated at a constant angular speed so that the laser beam scans in a predetermined angular range. The scanning beam is directed to pass through a scanning optical system having an fθ characteristic in a main scanning direction, which is a scanning direction of the beam. The laser beam is converged by the scanning optical system on a surface to be scanned to form a beam spot, which moves in the main scanning direction at a constant speed.
In order to cancel a shift of a position of the beam spot in an auxiliary scanning direction, which is perpendicular to the main scanning direction, on the surface to be scanned due to facet errors of reflection surfaces of the deflector, the laser beam emitted by the light source and collimated by a collimating lens is converged in the auxiliary scanning direction using a cylindrical lens arranged on an upstream side of the deflector so that the laser beam is converged in the auxiliary scanning direction on or in the vicinity of a reflection surface of the deflector. In such a case, the scanning optical system is configured such that the reflection surface of the deflector and the surface to be scanned have a conjugate relationship in the auxiliary scanning direction, while the scanning optical system converges the parallel light on the surface to be scanned in the main scanning direction.
As above, the scanning optical system generally has the fθ characteristic in the main scanning direction, and further the reflection surface of the deflector and the surface to be scanned have the conjugate relationship with respect to the scanning optical system.
Conventionally, such a scanning optical system consists of a plurality of glass lens elements. Each surface of the glass lens elements is processed to have an anti-reflection coating in accordance with a conventional method. With such a configuration, a ghost, which is formed as stray light reflected by the surfaces of the lens elements reaches the surface to be scanned, is insignificant.
Recently, however, plastic lenses are widely employed in order to reduce costs. When the plastic lenses are employed, due to technical problems and/or in view of manufacturing costs, there are cases where the anti-reflection coating is not formed on each surface of the plastic lens elements.
As a result, if the plastic lens elements are used as a part of or all of the lens elements of a scanning optical system, a possibility that the reflected light between the surfaces of the lens elements is generated becomes significant, which results in a significant effects of the ghost on the quality of the formed image of the laser beam printer or the captured image of the laser scanner.
Therefore, conventionally, various measures to deal with the ghost due to the surface-to-surface reflection in the scanning optical system have been suggested. One of such suggestions is disclosed in Japanese Patent Provisional Publication No. HEI 7-230051 of the present assignee. According to teachings of the publication, a predetermined surface of the plastic lens of the scanning optical system is decentered in the auxiliary scanning direction with respect to a reference scanning plane, which is defined as a plane on which the axis of the laser beam before incident on the scanning optical system scans, and a bow (curvature of a scanning line, which is a locus of a scanning beam on the surface to be scanned) due to the decentering of the predetermined surface of the plastic lens is compensated by decentering another lens surface in an opposite direction.
Embodiments of the above-identified publication having two lens elements are configured such that a spherical surface, cylindrical surface or toric surface is decentered, and therefore optical performance of the scanning system is insufficient. The scanning optical system consists of two lens elements are arranged at a position relatively close to the deflector. In order to compensate for the facet error, such a scanning optical system is required to have a relatively strong power in the auxiliary scanning direction. If a surface having a relatively simple surface (e.g., spherical, cylindrical or toric surface) which has a strong power is decentered, even though a bow is compensated, inclination of a scanning line and/or wavefront distortion may occur, which cannot be sufficiently suppressed. The inclination of the scanning line and/or the wavefront distortion may deteriorate the imaging quality.